By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
All photos by Bruce Rottink.
Slugs are a common sight at Tryon Creek State Natural Area (TCSNA). Just by chance, I’ve personally seen slugs at TCSNA every month except January and February. In November 2014, Tryon Creek Nature Guide Sharon Hawley wrote a Naturalist Note on slugs. You can read it here. This current note does not repeat the wonderful information that Sharon provided.
Banana slugs (Ariolimax columbianus) are a common species found at TCSNA. They are one of the slug species that are native to the Pacific Northwest. They were given their scientific name in 1851. These are not the slugs that typically cause problems in most people’s gardens.
Banana slugs are pretty low down in the food chain, and according to documentation on the web, are sometimes eaten by raccoons, garter snakes, ducks and even salamanders.
What is Slug Food?
Slugs eat a variety of things, including both living and dead vegetation. You can see slugs crawling on many types of plants. The picture below shows a slug on a stinging nettle (Urtica dioica) plant at TCSNA. I was a little surprised to see it there.
Below is a picture of a slug banquet I discovered occurring on the West Horse Loop Trail (and remembering the word “horse” is important here.)
Frequently I lead student nature hikes at the park, and oftentimes I’ve heard of kids attending an outdoor camp where they are encouraged to lick slugs. Just thinking of this picture ensures that I personally will never lick a slug. But enough of that!
The picture below shows a slug which climbed about 3 feet up the side of a tree near the Old Main Trail in order to get a taste of this mushroom. According to reports in the literature, mushrooms are one of their favorite foods.
The slug (of unknown species) in the photo below, is deeply exploring a trillium (Trillium ovatum) blossom. It appears that this slug may have chewed off substantial chunks of the trillium’s petals. Now it appears to be going out whole-hog for the core of the flower.
The banana slugs at TCSNA have lots of things to eat and explore.
Slugs are slow moving creatures, and I can’t imagine what predator they could successfully run away from. So slugs have chosen to employ camouflage. While the vast majority of the banana slugs at TCSNA have black spots, scientists who have studied the species have reported that a few, like the one below, do not.
Camouflage, with or without spots, means that you have to look like something else, or at least blend in to the environment. Below are several pictures of things I’ve seen on TCSNA trails that I’ve momentarily confused with a slug.
I can’t even begin to imagine how many banana slugs live at TCSNA. However, we must admit they are very successful, and make a contribution to the Park by helping turn plant material, both dead and alive, into soil.
Editor’s Note: Miss the Pacific Northwest rain? It’s been 48 days (June 21st) since measurable precipitation at Tryon Creek State Natural Area. Enjoy this post about rainfall in the forest!
Article by Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Mention “water” to anyone at Tryon Creek State Natural Area (TCSNA), and they will probably think of either the drinking fountain at the Nature Center, or Tryon Creek itself. However, we may need to consider other things in the park when someone brings up the topic of water. We can start by looking at the water cycle in the forest.
Here Comes the Rain
We are fortunate to be in an area with a pretty good rainfall. Sometimes it just drizzles, and sometimes it pours down. The first question is “where does the rain go?” Well, that depends on how heavy the rainfall is. This past April, I temporarily set up rain gauges at TCSNA when the forecast called for a rainy period for the next couple of days. I set up 2 rain gauges several feet apart under a large western hemlock (Tsuga heterophylla) and then placed a third rain gauge in a clearing less than 50 feet from the tree. I repeated this process with a large western redcedar (Thuja plicata). I checked the rain gauges after about a day of rain, and then again after 3 total days of rain. The results of both the redcedar and hemlock are combined and illustrated below:
It was astonishing to me that during the first 26 hours of rainfall totaling more than a third of an inch, that almost none of the rainfall penetrated the canopy of either tree. Okay, yeah, I know that when it starts to rain you head under a tree for shelter. But, I was surprised at how effective these under-tree shelters were. Even in the following two more days of rain, only a small portion of the water penetrated the canopy. For this three day event, only 18% of the total rainfall penetrated the canopy. No wonder there are few plants growing under mature trees of these two species.
I checked 2017 daily rainfall data collected by the City of Lake Oswego2 in downtown Lake Oswego, just a few miles from the park. The total annual rainfall was 53.13 inches. Based on my measurements during that one rain event, let’s assume that any daily rainfall of less than 0.35” will never hit the ground under these mature trees. In 2017, these light rains amounted to 25.9% of the total annual rainfall. Based on the information gathered in this study, none of that ever made it through the canopy. These means that plants growing under the canopy of redcedars and hemlocks experience a much different rainfall environment than other plants.
However, there can be lateral water movement in the soil once it hits the ground. To check that, I collected soil cores from beneath both the hemlock and the western redcedar. Under the redcedar the soil contained less water than in the surrounding areas beyond the redcedar’s canopy. For the hemlock, there was no difference between the under-the-canopy and outside-the-canopy soil water. This may have been due to the fact that the hemlock was growing on a significant slope, and the redcedar was growing in a flat area. Any rainfall uphill from the hemlock, probably traveled through the soil downhill to the hemlock.
And these aren’t the only species of plants that intercept the falling rain. Even our native Indian plum (Oemleria cerasiformis) seems to keep a lot of rain from ever hitting the ground, as seen in the picture below.
However, all is not lost. Numerous documents in the scientific literature point out that many plants can absorb water not just through their roots, but also through their leaves and needles.
An important function of the soil is to hold water for the plants to use. The forest at TCSNA is growing on soil that includes a significant layer of clay about 2-1/2 feet below the surface. Thus we see in some toppled over trees that the roots don’t go deep into the soil, but rather, tend to hit the clay layer and then begin to grow horizontally.
To determine how much water the soil holds, I used a soil corer to collect samples of only the top foot of soil at 21 locations at TCSNA. Thus this estimate of total water in the soil is VERY low, perhaps less than half of the water in the entire soil structure found at TCSNA. The approximate sampling locations are indicated on the map below.
I took the soil samples home and put them in plastic bowls to air dry. I weighed them periodically until they stopped losing weight. Then I calculated how much water was in the top 12 inches of soil at TCSNA. Then I carefully recalculated it 5 more times, because the answer astonished me. At the time I collected the soil samples, there was enough water in the top 12 inches of soil at TCSNA to fill 68 Olympic-sized swimming pools.
All plants need water to stay alive. As in humans, water is a key, and most often the dominant component of every plant. With the permission of TCSNA personnel, I collected the above ground parts of some plants, or parts of plants, and determined how much water they contained. The process was that I collected the plants in the forest, stuck them in a plastic bag, and immediately took them home and weighed them. Then I let them air dry in my garage. I periodically took the weights of each drying plant until the weight remained constant. Then I calculated the percent of water in the fresh plant. In a few cases the results were frankly surprising.
Latin Names not already noted: (Oregon grape, Mahonia nervosa; thimbleberry, Rubus parviflorus; swordfern, Polystichum munitum; horsetail, Equisetum sp.; red alder, Alnus rubra; English ivy, Hedera helix; waterleaf, Hydrophyllum tenuipes; jewelweed, Impatiens capensis;)
Plants contain a lot of water. Based on some samples I collected near the creek, if the entire park were covered in jewelweed about 4 feet tall (a typical mature height for this plant, the amount of water in the jewelweed would be more than enough to fill 1-1/4 Olympic sized swimming pools.
Both waterleaf and jewelweed will, under moist conditions, exude water from the edges of their leaves, especially on cool mornings. This is illustrated below (and no, it didn’t rain just before I took this picture).
The flip side of this is that waterleaf tends to wilt fairly easily on hot, dry days, as illustrated below.
In another spate of plant drying activity, I included the leaves of three species, and measured them on a schedule to compare how fast the leaves dried. The results are presented below.
The salal dried dramatically more slowly than either the elderberry or vine maple. This is not surprising because the salal leaves are much tougher than the other leaves. Salal is the only species of these three that holds its leaves over the winter.
It’s a wet, wet world
Water is unquestionably the dominant component of life on earth. The prominence of water in plants is documented above. Human beings, like me, and hopefully you, have been reported to contain somewhere between 55% and 60% water, with higher levels for infants. It is an amazing fluid that dissolves important nutrients, makes our cells turgid, and performs many other useful functions. Next time you see a rain cloud coming, be sure to step outside and say thanks.
1”Water, water everywhere,
And all the boards did shrink.
Water, water everywhere,
Nor any drop to drink.”
—- from The Ryme of the Ancient Mariner by Samuel Taylor Coleridge, 1797-1798
2 Thanks to Kevin McCaleb with the City of Lake Oswego for this data.
All photos by Bruce Rottink.
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
The forest at Tryon Creek State Natural Area (TCSNA) contains marvelous plants that we can enjoy at different seasons for different reasons. They range from the beautiful trillium (Trillium ovatum) blossoms in the early spring to the bright red leaves of the vine maple (Acer circinatum) in the fall. But these individual displays of beauty are transitory, as are the plants themselves. This is summed up in the Latin title of this note which means: “thus passes the glory of the world.”1
I started a phenology study in mid-2013. This involved, for the most part, identifying and tagging specific individual plants and monitoring their developmental stages each year. These stages were things like when I could first see the veins on the new leaves, and the first time I found open flowers on the plant. Now, just five years later, I am surprised at how many of those individual plants I was following have died in that short timeframe.
Plants are Persistent
As we realize, plants are persistent. In the photo below, you see the result of a very old injury to the trunk of a Douglas-fir (Pseudotsuga menziesii) growing at TCSNA. Long ago, the upright shoot of this tree was damaged or killed, and several side branches competed to take over the role of “leader.” The branches marked with red arrows lost the race to become leader and are now dead. The branch indicated by the blue arrow won, and became the leader so successfully, that it looks almost like it was always the leader. Several branches that were lower down the tree when the top was lost are marked with green arrows, and they remained horizontal.
Another example of a persistent plant is this mature black cottonwood (Populus trichocarpa) located alongside the Old Main Trail. Normally, mature black cottonwoods don’t have little branches popping out along the main trunk. However in this case, the reason can be seen in the wet dark seepage at the base of the tree. This tree appears to be infected with some microorganism (Fungus? Bacteria?) which is excreting a smelly fluid out of a crack in the tree. When Ranger Deb and I bored into the tree, the heartwood was definitely wet and smelly, evidence that it was decaying. In these cases, the tree doesn’t do such a good job of controlling the sprouting of the buds on the tree trunk.
Finally this Douglas-fir near Old Main Trail, which still has many green needles, sports numerous fungal fruiting bodies which indicate it is heavily decayed.
Plants are persistent, but…
Sometimes the trees have problems from which they never recover. The red alder (Alnus rubra) pictured below probably just aged out. Estimates of what constitutes “old age” for an alder varies from 60 years to a maximum of 100 years. Red alder is a species that likes full sun light and most frequently gets started on disturbed sites. So no surprise that we would find one this size dead.
A little more surprising is the dead western redcedar (Thuja plicata) pictured below.
This species is very shade tolerant, and under normal circumstances commonly lives several hundred years. So why is this relatively young tree dead? My best guess is based on the fact that this was found on the uphill side of the trail. Trails often serve as unintentional “dams” to the normal flow of underground water (great example: Old Main Trail near the Nature Center). A couple of years ago we had an extraordinarily rain-soaked winter season and I hypothesize that this cedar got “drowned out.” Yes, cedar frequently grows in wet-ish areas, but there is a limit to everything.
Individuals from several shrub species have recently died as well. This red elderberry (Sambucus racemosa) located just off the Old Main Trail (pictured below) is the plant that began my awareness of this topic and thus this whole article. This plant died before the recent winter with heavy rains. It was the first plant which was part of my multi-year phenology study that died. Additional walks around the park revealed many other dead elderberries. Again, it appears to be a fairly short-lived plant.
Perhaps the most dramatic die-off I’ve witnessed occurred near the upper section of the Red Fox Trail. Last year I noticed that many of the Indian plums (Oemleria cerasiformis) seemed to turn yellow and lose their leaves a little earlier than normal. This year, a relatively large number of them never leafed out. I laid out a 1/20 acre plot (a circle with a radius of 26.3 feet) and counted all of the Indian plum stems. I also measured their diameters at ground level. To the best of my ability, if I was able to determine that multiple stems were part of a single plant, I only measured the largest stem. Important confession: I chose an area with a very high density of dead stems. The results are summarized below:
In some cases these plants were quite large, both in height and diameter. I laid one of the stems on the sidewalk near the top of the Red Fox Trail to make it easy to see.
I selected a few of the larger Indian plums, and counted the annual rings at the base of the stem. They were between 15 and 19 years old.
As a final example, I’ve also noticed this year a number of dead salmonberry (Rubus spectabilis) in the forest. I don’t think this is the result of some climatic fluke or disease, because there are also a very large number of healthy salmonberries in every area where I’ve see a dead one. One example of a dead salmonberry is pictured below:
Below is a cross-section of stem from a dead salmonberry. Note the relatively large whitish pith in the center.
And let’s not forget the animals
Sometimes animals play important roles in the life of plants. A couple of years ago, beavers decided that a lot of the young cedar trees near Obie’s Bridge were ready to eat, and went in for the harvest. The results were evident by the number of chewed off stumps, like the one seen below.
“This too shall pass”2
The forest we see today is not the forest we will see tomorrow. Barring huge environment shifts, the major trend that we should expect is that much of our uplands forest will evolve to a predominantly redcedar-hemlock forest type. Douglas-firs will be relegated to a tiny role. Red alders may persist in some of the bottomlands near the creek. This of course will bring some shifts in the animals that inhabit our forest as well. It will be different, but still just as fascinating as it is today!
1Documentation on the web indicates this phrase was used as early as 1409 during the installation of the Pope.
2According to Wikipedia, this is an ancient Persian expression that worked its way into the English language sometime in the 1800s.